The present invention relates to a tracking error detecting apparatus which detects a tracking error of an optical spot that is obtained by radiating light on an optical recording medium.
As a method of obtaining a tracking control signal from an optical disk typified by a CD or a DVD in which information is recorded in the form of concavo-convex pits, a phase difference method has been employed in recent years.
As disclosed in Japanese Published Patent Application No. Hei.10-162381, the phase difference method is one which obtains a tracking error signal utilizing that when an optical spot deviates from the center of an information pit while the optical spot radiated on an information recording surface of an optical disk passes traversing on the information pit, a mapped image of the information pit on a photo detector (diffraction pattern) varies. That is, when the photo detector is divided in a track length direction for the information pit mapped image to see an output signal level according to respective accepted light quantities, the way of variation differs according to the direction and amount of the deviation of the optical spot from the center of the information pit. Therefore, by seeing the phase difference of the binarized signal which is obtained by binarizing the output of the photo detector with a prescribed level, a tracking error signal indicating the direction and amount of the deviation of the optical spot can be obtained.
A conventional method for detecting a tracking error will be described with reference to FIGS. 4 to 14.
FIG. 4 is a schematic diagram illustrating a main configuration of an optical pickup part 100 in FIG. 4, an astigmatic method is employed for a detection of a focus error signal.
A luminous flux radiated from a light source 1, such as a semiconductor laser is converted into a parallel beam at a collimator lens 3, goes through a half mirror 6, is converged by an objective lens 4, and is radiated on an information recording surface 51 on an optical recording medium (such as an optical disk) 5 as a small optical spot. A reflected light of the optical spot goes through the objective lens 4, has its optical path inflected in the right hand direction of the figure by the half mirror 6, and reaches a photo detector 2 through a convex lens 61 and a cylindrical lens 62 to be a convergent light having two focuses characteristic in the astigmatic method. Information on the optical recording medium 5 is recorded by an information pit line having unevenness.
Next, a description will be given of a method of obtaining a tracking error signal which indicates a positional deviation of the optical spot in a vertical direction against the pit line (track) in the information recording surface, by utilizing a diffraction pattern of light generated when the optical spot passes traversing on the pit.
An intensity distribution pattern (far field pattern) of the reflected light quantity of the optical spot changes according to the position of an information pit on which the optical spot passes traversing,
FIGS. 5(a)-(c), 6(a)-(c), and 7(a)-(c) are diagrams exemplifying changes of the far field pattern of the reflected light quantity when the optical spot passes traversing on the pit. (a) of each figure is a diagram illustrating the physical relationship between an optical spot 12 and an information pit 13 (the center of the information pit 13 is described by a dotted line), and the optical spot 12 proceeds on the information pit 13 in a direction of arrows. (b) of each figure shows the transition of the intensity distribution pattern (far field pattern) of the reflected light quantity on the photo detector 2, and the three patterns shown in (b) of each figure respectively represent patterns when the optical spot 12 is at three positions shown in (a). (c) of each figure shows two signals obtained from the photo detector 2. Further, the photo detector 2 has photo acceptance units 2a-2d, respective twos being arranged vertically and horizontally, and the two signals obtained in (c) of each figure are ones which are obtained as a result of adding signals, that are obtained from the four photo acceptance units 2a-2d, for the photo acceptance units in a diagonal direction, respectively (i.e., 2a+2d and 2b+2c).
For example, as shown in FIG. 5(a), when the optical spot 12 passes traversing on the left of the center of the information pit 13 in the direction of movement, the pattern changes to rotate clockwise as shown in FIG. 5(b), resulting in two signals out of phase as in FIG. 5(c).
As shown in FIG. 6(a), when the optical spot 12 passes traversing on the center of the information pit 13, that is, the center of a track, the pattern changes symmetrically as in FIG. 6(b), resulting in two signals in phase as in FIG. 6(c).
As shown in FIG. 7(a), when the optical spot 12 passes traversing on the right of the center of the information pit 13 in the direction of movement, the pattern changes in a counterclockwise direction as shown in FIG. 7(b), resulting in two signals out of phase as in FIG. 7(c).
As described above, it is proved that the transition of the field pattern changes when the optical spot deviates from the center of the information pit. The phase difference method is the one that utilizes the changes of the far filed pattern so as to detect a tracking error signal. That is, the method comprises comparing phases of two adding signals obtained from the photo detector 2, and detecting the degree of phase advancement or delay, thereby recognizing a positional deviation between the optical spot 12 and the information pit 13.
A conventional tracking error detecting apparatus will be described with reference to FIGS. 8 and 9(a)-(h). FIG. 8 is a block diagram illustrating an example of a tracking error detecting apparatus which detects a phase difference to detect a tracking error signal, and FIGS. 9(a)-(h) diagrams of illustrating waveforms of signals denoted by (a)-(h) in FIG. 8. Further, FIGS. 9(a)-(h) diagrams of waveforms in a case where according to a passage of time, the optical spot 12 passes traversing on the information pit 13, crossing from the left side to the right side in the direction of movement, that is, changing from the state in FIGS. 5(a)-(c) to that in FIGS. 7(a)-(c).
The photo detector 2 has the photo acceptance units 2a, 2b, 2c, and 2d, respective twos being arranged vertically and horizontally, and detects optical signals to project into respective units as a photoelectric current. The detected photoelectric current is converted into voltage signals by current/voltage conversion circuits 7a, 7b, 7c, and 7d, respectively.
Next, adder 8a and 8b adds signals which are obtained from two pairs of units in a diagonal direction of the photo detector 2, for respective pairs. That is, an adder 8a adds outputs of the current/voltage conversion circuits 7a and 7c, and an adder 8b adds outputs of the current/voltage conversion circuit 7b and 7d. Two adding signals (a) and (b) become waveforms shown in FIGS. 9(a) and 9(b), respectively.
The adding signals (a) and (b) pass through binary circuits 9a and 9b so that binary signals (c) and (d) are obtained, respectively.
A phase difference detector circuit 10 detects a phase difference of rise or fall of the binary signals (c) and (d). In the circuit configuration shown in FIG. 8, a phase difference of fall is detected employing D-type flip flops (D-FF) 101a and 101b. The D-FFs 101 a and 101b have input terminals D, clock input terminals T, reset input terminals R, and output terminals Q and Q-, and when an input of the reset input terminal R is at logic level, an output of the output terminal Q is unconditionally at level, and when an input of the reset input terminal R is at logic level, a signal the logic level of which is the same as that applied to the input terminal D is outputted from the terminal Q at the fall of the clock input terminal T, xe2x80x9cHxe2x80x9dxe2x86x92xe2x80x9cLxe2x80x9d. That is, the D-FFs 101a and 101b detect phase differences of the binary signals (c) and (d) to obtain time difference pulses (e) and (f), respectively. The time difference pulse (e) is outputted from the output terminal Q of the D-FF 101a, and the time difference The time difference pulses (e) and (f) are converted into a pulse-width modulation signal (g) at a difference detector 102, which further passes through a low-pass filter 11 to be an analog tracking error signal (h).
FIG. 10 illustrates a waveform of the tracking error signal (h) obtained when the tracking error signal is observed for plural tracks. When paying notice to a neighborhood of a specific track, the tracking error signal (h) obtained by the tracking error detecting apparatus shown in FIG. 8 is a nearly linear signal which is at a zero level when the optical spot is on the center of the track, and which, when the optical spot deviates right and left therefrom, has polarities according to the direction of the deviation. When observing the tracking error signal for plural tracks, the above-described linear signal waveform appears for each track, and when the optical spot is between tracks, a zero level is obtained, whereby as a whole, a serrate waveform repeated for each track is obtained as shown in FIG. 10.
In order to perform a tracking servo control employing the tracking error signal which appears as a serrate waveform repeatedly for each track with the polarity as in FIG. 10, a tracking servo control system is constructed so as to drive the objective lens 4 by a means generally referred to as a tracking actuator according to the positive and negative of the tracking error signal.
Further, since the conventional phase difference method detects the tracking error signal from respective pits on which the optical spot passes traversing, it is likely to be affected by the shape or depth of the pit, whereby an offset is generated in the tracking error signal when the objective lens 4 is FIGS. 11(a)-(c) and 12(a)-(c) are diagrams illustrating principles of offset generation when detecting the tracking error signal by the phase difference method, and FIGS. 11(a)-(c) shows a case where the depth of the information pit 13 is xcex/4 (xcex: wavelength of light source), while FIGS. 12(a)-(c) shows a case where the depth of the information pit is other than xcex/4. In the figures, (a) Figure (a) illustrates an intensity distribution pattern (far field pattern) of the reflected light quantity on the photo detector 2 when the objective lens 4 does not move, Figure (b) illustrates an intensity distribution pattern (far field pattern) of the reflected light quantity on the photo detector 2 when the objective lens 4 moves, and Figure (c) illustrates a tracking error signal obtained. Further, Figures (a) and (b) illustrate cases where the optical spot 12 passes traversing on the center of the track and is located at the end of the information pit 13.
As shown in FIG. 11(a), in a case where the depth of the information pit 13 is xcex/4 and the objective lens 4 does not move, patterns which appear in a first area (2a+2d) into which the photo acceptance units in a diagonal direction of the photo detector 2, 2a and 2d, are combined and in a second area (2b+2c) into which the other photo acceptance units in a diagonal direction, 2b and 2c, are combined are the same. In addition, as shown in FIG. 11(b), even when the objective lens 4 moves and the optical spot 12 on the photo detector 2 moves, the phase difference between signals outputted from the first area (2a+2d) and the second area (2b+2c), respectively is zero, as long as the optical spot 12 is on the center of the track. Therefore, as shown in FIG. 11(c), tracking error signals, the waveform patterns of which at the parts indicated by arrows A and B are the same, can be obtained.
Meanwhile, as shown in FIGS. 12(a)-(c), in a case where the depth of the information pit 13 is other than xcex/4, a phase difference between signals outputted from the first area (2a+2d) and the second area (2b+2c) may be generated. As shown in FIG. 12(a), when a reflected light of the photo detector 2 does not move, there is no phase difference between the first area (2a+2d) and the second area (2b+2c), resulting in a tracking error signal of zero, but on the other hand, when the objective lens 4 moves as shown in FIG. 12(b), there is generated an unbalance and thus, the phase difference between the first area (2a+2d) and the second area (2b+2c), resulting in a generation of an offset in a tracking error signal. Therefore, tracking error signals, the waveform patterns of which at the parts indicated by arrows A and B in FIG. 12(c) are different, are obtained. When an offset is generated, it is impossible to perform tracking toward the center of the track, thereby deteriorating the quality of a reproduced waveform.
To solve the above-mentioned problems, a tracking error detecting apparatus as shown in FIG. 13 is proposed. In FIG. 13, the same configurations as those shown in FIG. 8 are denoted by the same reference numerals.
The above-described tracking error detecting apparatus adjusts the phases of the signals outputted from the photo detector 2 by employing delay circuits 14a and 14b, and thus, the offset of a phase difference tracking error signal can be canceled, thereby performing tracking toward the center of the track.
However, in case of the tracking error detection by the conventional method, the tracking error signal is detected by an analog signal processing, whereby it is not suited for doubling the speed of an optical recording/reproducing apparatus and for enhancing a density of an optical recording medium.
Here, problems due to doubling of speed and enhancing of density will be described.
While the tracking error detecting apparatus by an analog signal processing shown in FIG. 13 constructs an all pass filter with the delay circuits 14a and 14b for canceling an offset, and obtains the delay amount by group delay of the filter, in a case where the optical recording/reproducing apparatus doubles its speed, a channel route of read data of the optical recording/reproducing apparatus is different, whereby the required amount of delay changes considerably, and thus, the optimization of the delay circuits is required.
Further, when the recording density of the optical recording medium is high, the high frequency component of a read signal obtained from the photo detector 2 is attenuated, whereby it is impossible to detect a phase difference signal correctly.
As a means to solve this, a tracking error detecting apparatus a as shown in FIG. 14 is proposed. In FIG. 14, the same configurations as those shown in FIG. 8 are denoted by the same reference numerals, and their detailed descriptions will be omitted.
The tracking error detecting apparatus shown in FIG. 14 performs a high frequency emphasis toward two sum signals of the photo detector 2, (2a+2d and 2b+2c), which are obtained by the adders 8a and 8b, by waveform equalization filters 15a and 15b, and binarizes them by the binary circuits 9a and 9b subsequently, so as to obtain a phase difference signal, whereby attenuation of the high frequency component due to high density can be compensated.
However, since the waveform equalization filters 15a and 15b are composed of analog FIR filters, an all pass filter is required to compose a delay part of the FIR filter, whereby a problem described in the above-mentioned speed doubling occurs. In addition, when the recording density is different, the characteristics of high frequency emphasis required are different, whereby it is impossible to cope by the above-described tracking error detecting apparatus when further density enhancing is performed.
As described above, it is difficult to cope with speed doubling in an optical recording/reproducing apparatus and density enhancing of an optical recording medium by the conventional tracking error detecting apparatus which performs the tracking error detection by an analog processing. Further, since the conventional tracking error detecting apparatus has a large number of configurations involving the analog processing, it is difficult to unite the tracking error detecting apparatus with peripheral digital signal processing parts.
The present invention is made to solve the above-mentioned problems and has for its object to provide a tracking error detecting apparatus which can cope with speed doubling of an optical recording/reproducing apparatus and density enhancing of an optical recording medium in a small size and at low cost.
A tracking error detecting apparatus according to the present invention comprises: a photo detector for receiving reflected light of the optical spot and outputting photoelectric current according to the photo acceptance quantity; current/voltage conversion circuits for converting the photoelectric current of the photo detector into voltage signals; signal generators for generating two signal series, the phases of which change each other according to a tracking error of the optical spot, from the voltage signals; analog-digital converters for discretizing the two signal series to obtain first and second digital signal series; interpolation filters for performing interpolation processing toward the first and second digital signal series respectively; zero cross point detector circuits for respectively detecting zero cross points of the first and second digital signal series interpolated by the interpolation filters; a phase difference detector circuit for detecting a phase difference between the zero cross point of the first digital signal series and the zero cross point of the second digital signal series; and a low-pass filter for performing band restriction toward the detected phase difference to obtain a tracking error signal.
With the tracking error detecting apparatus of this configuration, a tracking error can be detected by digital signal processing, whereby it is easy to unite the signal processing after the ADC with peripheral digital signal processing parts. Further, required analog signal processing blocks can be reduced considerably. In addition, it is possible to cope with speed doubling in an optical recording/reproducing apparatus and density enhancing of an optical recording medium, whereby an optical recording/reproducing apparatus can be provided in a small size and at low cost.
Further, another conformation of the tracking error detecting apparatus according to the present invention comprises: a photo detector for receiving reflected light of the optical spot and outputting photoelectric current according to the photo acceptance quantity; current/voltage conversion circuits for converting the photoelectric current of the photo detector into voltage signals; analog-digital converters for discretizing the voltage signals to convert into digital signals; interpolation filters for performing interpolation processing toward the digital signals; signal generators for generating first and second digital signal series, the phases of which change each other according to a tracking error of the optical spot, from the signals obtained at the interpolation filter; zero cross point detector circuits for detecting zero cross points of the first and second digital signal series respectively; a phase difference detector circuit for comparing phases of the zero cross point of the first digital signal series and the zero cross point of the second digital signal series, so as to detect a phase difference; a low-pass filter for performing band restriction toward the detected phase difference; an offset detector circuit for detecting an offset in a tracking error signal from the output signal of the low-pass filter; and a factor setting circuit for setting a factor of the interpolation filter according to the detected offset.
With the tracking error detecting apparatus of this configuration, a tracking error can be detected by digital signal processing, whereby it is easy to unite the signal processing after the ADC with peripheral digital signal processing parts. Further, required analog signal processing blocks can be reduced considerably. In addition, it is possible to cope with speed doubling in an optical recording/reproducing apparatus and density enhancing of an optical recording medium, whereby an optical recording/reproducing apparatus can be provided in a small size and at low cost.